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Some Ramblings on Image Size and Aspect Ratios

Some thoughts on the size of your sensor and the size of your images.
It is pertinent if you are using online or external print services to make your prints.
Not as important if you are printing at home (with home printing you don’t have to stick to conventional print sizes)
If this confuses you – and it probably will! – it took me a while to realise the significance of aspect ratio versus resolution versus finished print size, then come along to the club next Thursday to discuss it.

First lets consider the sizes of todays common sensors, the crop DX and full frame FX.
Although Megapixel count will indicate the relative size of your image, what matters is the RESOLUTION of your sensor.
A typical DX sensor with a 24 MPixel sensor will have a resolution of something like 6000×4000 pixels.
A good quality FX sensor with 36 MPixels will have a resolution of 7360×4912.
The COMMON FACTOR in both sensors DX and FX is the ASPECT RATIO.
Both sensors produce, straight from the camera, images with an aspect ratio of 3:2.
Simple maths proves this:
6000×4000 : 6000/3=2000, 4000/2=2000 – aspect ratio = 3:2
7360×4912 : 7360/3=2454, 4912/2=2456 – aspect ratio = 3:2

An important thing to consider when composing your image in the viewfinder is that if you are going to print your image for a conventional size print – then you have to imagine a 4:5 or a 4:3 ratio image in your 3:2 viewfinder!

So what is the significance of this bit of information?
Your sensor is 3:2 aspect ratio
Most common conventional print bureau sizes are 4:5 or 4:3 aspect ratio.

To get all of the information you want to show in your image – you are going to have to crop the 3:2 ratio to 4:5 ratio so it will fit the printers common print size specifications.

So the saying “fill the frame with your image” might not hold true for this scenario. You have to leave enough room around the outside of your image when framing it to enable you to crop it to the correct aspect ratio for printing.

Do This – find out the size of your sensor in pixels. Divide each measurement by 300 (or 240 or whatever resolution you want to print your image) to find out the maximum print size. Decide what dimensions you want your finished image to be. (Lightroom has a great tool – an overlay – that you use in the CROP menu to accurately size your image to the appropriate aspect ratio.) Photoshop will let you specify the ratios or dimensions and ppi/dpi setting in its crop tool.

The bottom line is – if you are going to be printing/finishing an image to a certain set of dimensions, consider the aspect ratio of the finished dimensions compared to the aspect ratio of the native sensor, and leave room around your intended main subject to allow cropping – it will save a lot of grief deciding which bits you are going to “crop out” to fit the finished print size.